How do I know if Nest is power stealing

[Charlton]

Hi, I am trying to figure out if my 3rd Gen Nest thermostat is power stealing from my furnace. Any ideas on how I can check that?

also, I’ve looked into installing a C wire for it, but there appears to be a white wire connected into the hvac panel (see attached)

(this thermostat is on the 1st level and connects to the furnace in the basement. Current set up is R, W, Y and G wires connected to the thermostat)

please help and advise.

thank you!

 

 

 

[Houptee]

Is there a spare wire at the Nest maybe blue not connected in the cable in the wall that you can connect to C?

If yes then you need to go to the furnace and connect the same color wire to the C with the white wire in the same screw. You have a white there now that goes out to the outdoor AC unit, they are using red and white to go outside with red acting as Y (see red is in the same screw terminal with Y1 then it goes outside to AC unit and white returns back to C to make a completed circuit.)

So hopefully you have a spare color in the cable to the Nest to use for a C wire. Otherwise if possible buy a roll of 8 conductor thermostat cable at lowes or home depot and run a new cable and use blue for C wire.

[Charlton]

I do have a blue unused wire available at the thermostat. is there a way I can check if it’s power stealing from my furnace before connecting the blue wire?

[Houptee]

The Nest uses the 24 v power to keep it powered up and not drain the battery, so the Nest does use some power even if it has a C wire connected.

The C wire provides a stable 24v all the time to keep it from dropping off the wifi system and killing the battery so if you have the extra wire hook it to C at both ends.

[Charlton]

Ahhh got it. So I’ve tried using the Nest with just R, W, Y and G wires and it looks like it’s giving it enough power? I am just not sure if it will “run” my furnace/cooling for short periods just to keep it powered up. Any advice?

[Houptee]

No it doesn't run the equipment just to charge the battery.

It uses a tiny bit of power from the wires to stay powered up and keep it online.

It uses the 24v transformer for the power.

[Charlton]

Thank you Houptee! 
if that’s the case, am I right to say I can keep to the 4 wire setup and not have the C wire?

I have tried with a multimeter at the thermostat between R and Y/G/W, and they give around 28V. Does it mean that it’s good as is?

pardon my numerous questions but this is very helpful!

[Houptee]

You can leave it alone if you never had any messages about power problems or battery low.

The Learning model can usually get its C from the other wires but the non learning model needs a C wire.

[Charlton]

thank you so much Houptee, this is very helpful! 
I am assuming this wouldn’t damage the furnace as compared to having a C wire?

[Jake]

Hey Charlton,

Thanks so much for the help Patrick_Caezza, and Houptee.

I wanted to check in and see if you managed to see Patrick_Caezza's post. Please let me know if you have any questions from here. I would be happy to assist, and make sure you are good to go.

Best regards,
Jake

[Charlton]

Yes I do want to thank both of them for their help. Very appreciative. Please close this. Thanks

[Jake]

Hi Charlton,

I am glad to hear everything is good to go from here. Per your request, I am going to go ahead and lock the thread at this time. If you run into any questions from here, please feel free to create a new post.

Best regards,

Jake

[Patrick_Caezza]

It is better to have a C wire connected than not.

The Nest "Power Stealing" method to keep the internal battery charged does always keep it fully charged, and after some time, the battery starts to fail, and you start having problems with the thermostat.

With the C wire installed, the thermostat will always use the power from the system and only use the internal battery when needed.

Every Nest thermostat I have installed is always installed with a C wire, and I have never received a complaint about power issues.